In recent years, nonaqueous electrolyte secondary batteries typified by a lithium ion secondary battery having a large capacity have been developed to be used for, for example, vehicles such as hybrid electric vehicles (HEVs) and electric vehicles (EVs) and for large-scale storage battery systems.
A lithium ion secondary battery contains a material capable of absorbing and desorbing lithium ions, such as a carbon material and a silicon material, as a negative electrode active material, and a lithium transition-metal oxide such as LiCoO2, LiNiO2, and LiMn2O4, as a positive electrode active material, and includes an electrolyte in which a lithium salt as a solute is dissolved in an organic solvent.
In a lithium ion secondary battery in an overcharged state, lithium is excessively extracted from the positive electrode, and lithium is excessively inserted into the negative electrode. Thus, both the positive and negative electrodes become thermally unstable. The thermally unstable positive and negative electrodes cause an organic solvent contained in the electrolyte to decompose. This invites a sudden exothermic reaction, and the battery abnormally generates heat to impair the reliability.
To solve such a problem, for example, a lithium ion secondary battery has been developed (see JP-A-2004-134261) in which at least one of biphenyl, cyclohexylbenzene, and diphenyl ether is added as an overcharge inhibitor to the electrolyte so as to prevent the increase in temperature if the battery is overcharged.
Another lithium ion secondary battery has been developed (see JP-A-2001-015155) in which an alkylbenzene derivative or a cycloalkylbenzene derivative having a tertiary carbon atom adjacent to the phenyl group is added to an organic solvent in the electrolyte so as to ensure the safety of an overcharged battery without adversely affecting battery characteristics such as low-temperature characteristics and storage characteristics.
If the lithium ion secondary battery is overcharged, an additive such as cumene, 1,3-diisopropylbenzene, 1,4-diisopropylbenzene, 1-methylpropylbenzene, 1,3-bis(1-methylpropyl)benzene, 1,4-bis(1-methylpropyl)benzene, cyclohexylbenzene, and cyclopentylbenzene starts to decompose and generates gas. Concurrently with this, a polymerization reaction starts to generate polymerization heat. If the overcharging continues in this condition, the amount of the gas increases, and after 15 to 19 minutes from the start of the overcharging, a current interruption sealing plate is activated to interrupt the overcharge current. As a result, the battery temperature gradually decreases.
The techniques described in JP-A-2004-134261 and JP-A-2001-015155 achieve a certain effect. However, the nonaqueous electrolyte secondary battery having a large capacity used for vehicles and the like requires further improved reliability.